In games, "particles" and "particle emitters" refer to a whole class of behaviors that are usually used for special effects and flavor. The "emitter" is the source and manager of the actual "particle" objects that are spewed out and/or floating around. FlxParticle is just an extension of FlxSprite, and FlxEmitter is just an extension of FlxGroup, so a particle system in Flixel isn't really that different from any normal group of sprites. It just adds some special behavior for creating and launching particles, and the particles themselves have some optional, special behavior to bounce more believably in platformer situations. FlxEmitter also has built-in variables that let you specify velocity ranges, rotation speeds, gravity, collision behaviors, and more.